254 - Characterizing the Development of Compound Flood Transition Zones Along Coastal Urban Domains

Urban communities are increasingly exposed to compound flooding events from climate change (i.e., intensifying hydrology, higher oceanic water levels, increasing waves). Coastal and estuarine communities are particularly vulnerable to more frequent and intense compound events because they are exposed to marine, fluvial, and pluvial flows. The Federal Emergency Management Agency (FEMA) provides flood analysis mapping guidance for various compound events; however, the current standards rely upon a simple superposition approach of each flood driver and neglect nonlinear interactions (such as high tailwater conditions) within transition regions. This study explores flood transition zone physics in Pacific coast urban communities by explicitly accounting for marine water levels, riverine discharge, rainfall, waves, and their interactions with flood control infrastructure. Dynamic hydrologic and marine interactions are explored using a validated nonlinear shallow water model (e.g., AdH, Delft3D). Preliminary results suggest west coast storm surge impacts are minimal, while areas exposed to intense precipitation and subsequent pluvial flow are highly sensitive within the transition region. Sea level fundamentally alters transition zone characteristics, particularly in low-lying communities with existing flood control infrastructure (i.e., sea walls, storm water drainage systems).